Medical imaging data is conventionally represented by a two- or a three-dimensional data set in which each pixel or voxel has a data value which relates to a physiologically significant aspect of an imaged region. In order to display a medical image the data values of a selection of the pixel or voxels are mapped to a two-dimensional array of image values. Each image value in the two-dimensional array typically represents the intensity of a color, or a grayscale intensity depending on the type of image to be displayed. Conventional display monitors used in the display of such medical images are typically capable of displaying a wide range of image values in both color and monochrome.
A limitation that arises in the display of such images however is that when the full range of data values in an image is mapped to the display monitor, physiological features which give rise to small changes in data values result in small changes in shade or color that may be hidden in the displayed image.
In X-ray for example, Hounsfield units (HU) are used to represent the radiological density of matter. Conventionally air has a radiological density of −1024 HU, water has a radiological density of 0 HU and that for bone may lie in the range 700 to around 3000 HU. Thus in order to display the full range Hounsfield units in a medical image, approximately 4000 discrete pixel values should be displayed. Conventionally such radiological images are displayed in grayscale. The human eye, however, is generally accepted as being capable of discerning a lower number of grayscale steps, or shades. At a typical display brightness values of 500 cd/m2 an untrained eye is generally considered capable of discerning approximately 720 discrete grayscale steps (Digital Imaging and Communications in Medicine (DICOM), Part 14: Grayscale Standard Display Function, PS 3.14-2011, P11-15). A trained radiologist performs rather better and is typically considered capable of discerning between 800 and 1000 discrete levels (Blume H., Muka E., “Hard Copies for Digital Medical Images: an overview”, Proc. SPIE 2413, Color Hard Copy and Graphic Arts IV, 206, Apr. 27, 1995). Therefore the concurrent displaying of all possible HU values as different shades results in some shades being indiscernible from others. Consequently image features giving rise to small changes in grayscale values may be hidden in images in which the full scale of HU values is displayed concurrently.
Similar issues arise in the display of medical images from other medical imaging fields including in CT, X-ray, PET, SPECT, MR and ultrasound in which the desire to display a wide range of data values leads to the hiding of image features that result from small changes in data values.
A conventional method of resolving these issues is to use the level-and-window technique in which data values within a particular range, determined by the width of a so-called window, and centered about a particular data value, the level, are mapped to the full range of pixel values available in a display. Data values at the lowest end of the range are typically mapped to black, and data values at the highest end of the range are typically mapped to white. The level and window may be adjusted in order to permit a user to dynamically focus on a particular physiologically-interesting range of data values, such as a bone window or a tissue window.
A drawback of the level and window technique is that a radiologist is frequently interested in several windows relating to different physiological ranges at the same time. The radiologist must then set-up different windows for each medical image. In one solution to this issue, U.S. Pat. No. 6,993,171B1 relates to a method of displaying colorized two-dimensional medical images of human anatomy and discloses a method of assigning a predetermined range of colors to computed tomography images for enhanced visualization. Each color band may be mapped to a distinct sub-range of radiological density values, the sub range being unique to a particular tissue type.
In another solution, patent application WO2010/009040A1 relates to a method for separating diagnostic content of x-ray images from non-diagnostic content in order to achieve an optimum image reproduction and windowing. In particular, embodiments are disclosed for displaying a plurality of radiological density images in a single image viewing window by overlaying gray scale images of each radiological density image in a single display window.
In the assessment of medical images a medical expert is frequently faced with a further desire to compare medical images from different imaging modalities. One imaging modality may for example show structural image features and a second imaging modality may for example show functional image features. By comparing the two images an improved diagnosis is achieved because the physical regions, such as the organs, to which the functional data relates, are more readily identified. In one method disclosed in patent application WO2010/084390 a second image is color coded with respect to an intensity spectrum with a portion of the intensity spectrum set to be transparent to generate a color coded second image. This image is combined with a first image to generate a fused image, and the fused image is displayed.
However, these solutions suffer from the drawback that when displaying medical images from different imaging modalities, some image features remain hidden, confounding image analysis.